Add production features: slog adapter, scan helpers, slow query logging, pool stats, tracer passthrough, test tx isolation

- slog.go: SlogLogger adapts *slog.Logger to dbx.Logger interface
- scan.go: Collect[T] and CollectOne[T] generic helpers using pgx.RowToStructByName
- cluster.go: slow query logging via Config.SlowQueryThreshold (Warn level in queryEnd)
- stats.go: PoolStats with Cluster.Stats() aggregating pool stats across all nodes
- config.go/node.go: NodeConfig.Tracer passthrough for pgx.QueryTracer (OpenTelemetry)
- options.go: WithSlowQueryThreshold and WithTracer functional options
- dbxtest/tx.go: RunInTx runs callback in always-rolled-back transaction for test isolation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.cursorrules

@@ -23,8 +23,11 @@ You are working on `git.codelab.vc/pkg/dbx`, a Go PostgreSQL cluster library bui
   - `tx.go` — RunTx, RunTxOptions, InjectQuerier, ExtractQuerier
   - `errors.go` — IsRetryable, IsConnectionError, IsConstraintViolation, PgErrorCode
   - `config.go` — Config, NodeConfig, PoolConfig, RetryConfig, HealthCheckConfig
-  - `options.go` — functional options (WithLogger, WithMetrics, WithRetry, WithHealthCheck)
-- `dbxtest/` — test helpers: NewTestCluster, TestLogger
+  - `options.go` — functional options (WithLogger, WithMetrics, WithRetry, WithHealthCheck, WithSlowQueryThreshold, WithTracer)
+  - `slog.go` — SlogLogger adapting *slog.Logger to dbx.Logger
+  - `scan.go` — Collect[T], CollectOne[T] generic row scan helpers
+  - `stats.go` — PoolStats aggregate pool statistics via Cluster.Stats()
+- `dbxtest/` — test helpers: NewTestCluster, TestLogger, RunInTx
 
 ## Code conventions